1. Field of the Invention
The present invention relates to an image forming method and apparatus and, more particularly, to an electrophotographic image forming apparatus such as a laser beam printer or copying machine, a control method therefor, and a process cartridge.
2. Related Art
Image forming apparatuses for forming images on the basis of image signals sent from host computers include various types of apparatuses, e.g., electrophotographic, ink-jet, and thermal image forming apparatuses.
Recently, of these apparatuses, electrophotographic image forming apparatuses have become greatly popular because of their advantages, e.g., high printing speed, high image quality, quietness.
For example, an electrophotographic image forming apparatus based on a laser exposure scheme includes an image forming circuit for converting image data sent from a host computer into bitmap data, and an image signal generating means for generating a laser ON/OFF signal on the basis of the bitmap data. This laser ON/OFF signal is input to a laser driving circuit to drive a semiconductor laser diode so as to emit light, thereby exposing a photosensitive body surface and forming an image thereon.
When the photosensitive body surface is irradiated with a laser beam corresponding to image data, an electrostatic latent image is formed. This image is developed and visualized by toner (recording agent). The visualized toner image is transferred onto a recording sheet and fixed by a fixing apparatus. In this manner, a user can obtain a desired image.
Image density adjustment, e.g., changing the thickness of a character in accordance with user""s preferences, is performed by changing the developing contrast obtained by the developing apparatus.
If, however, a line image is printed, a portion corresponding to black image data has a so-called mountain-like cross-section. That is, the height of toner deposited on end portions of this line image portion is small, whereas the height of toner deposited on a central portion is large. This poses the following problems.
(1) The first problem is the phenomenon of a smeared image trailing edge in which the moisture contained in a paper sheet vaporizes when it passes through the fixing apparatus, and the water vapor is trapped in a portion where the height of toner is large. As a result, the toner bubbles over.
(2) The second problem is the occurrence of a solid image loss defect, which is caused when the toner on a line portion is pressed against the photosensitive drum by the pressing force of the transfer roller.
(3) The third problem is the offset phenomenon, a phenomenon in which toner cannot be completely fused because of an excessive amount of toner fed into the fixing apparatus, and toner is peeled off from a recording sheet by the fixing roller.
(4) A fourth problem is a deterioration in sharpness of a character that is caused when the toner of a character is smashed by the pressing force of the fixing roller because of its large height, and the contour position moves.
(5) The fifth problem is scattering of a character portion, which is caused when the amount of charge is small relative to the amount of toner developed because the developed toner image has difficulty in being faithfully influenced by a transfer electric field.
Under the circumstances, a printing method of decreasing the amount of toner consumed by decreasing the height of toner in printing a line image is disclosed in Japanese Patent Application Laid-Open No. 9-085993. In this method, the density data of the contour portion of a line image is used without any change, but the density data of an inner portion is corrected into low-density data, thereby decreasing the height of toner on a central portion of the line.
In the printing method disclosed in the above reference, however, when a user changes a density adjusting value (developing contrast) for the purpose of image density adjustment, line width adjustment, or the like, image data correction processing cannot be satisfactorily performed, resulting in the following problems. If image data is adjusted to a high image density to increase the line width, since the height of toner also increases, a sufficient decrease in the height of toner cannot be attained due to insufficient thinning-out. This may cause image problems (1) to (5). In contrast to this, if image data is adjusted to a low image density, since the height of toner also decreases, the height of toner excessively decreases due to excessive thinning-out, resulting in a poor line image.
In addition, since variations in line width occur due to variations of photosensitive bodies of process cartridges and developing characteristics and changes in the number of printing operations in the cartridges, when the line width is large, the height of toner does not decrease sufficiently due to insufficient thinning-out. This may cause problems like problems (1) to (5) described above.
On the other hand, if the line width is small, the height of toner decreases too much due to excessive thinning-out, resulting in an unclear line image. As described above, variations of process cartridges lead to inappropriate image data correction processing.
The present invention has been made in consideration of the above problems, and has as its object to provide an image forming apparatus, a control method therefor, and a process cartridge which can suppress the amount of toner consumed in forming a visible image low, and form a good image by recording even a line image sharply while reducing scattering, smeared image trailing edges, offsets, hollow defects, and the like even when a user changes a density adjusting value (developing contrast) for the purpose of image density adjustment, line width adjustment, or the like, or even due to variations in cartridges, the number of printing operations, and the amount of toner used.
According to the present invention, there is provided an image forming apparatus comprising: detecting means for detecting an edge of image data; image data correcting means for correcting the image data inside the edge detected by the detecting means so as to decrease a density of the image data; forming means for forming an electrostatic latent image on a photosensitive body on the basis of the corrected image data; acquiring means for acquiring an instruction value for giving an instruction to correct a density contrast; and developing means for developing the electrostatic latent image formed on the photosensitive body on the basis of the instruction value acquired by the acquiring means, wherein a correction condition for image data corrected by the image data correcting means changes in accordance with the instruction value acquired by the acquiring means.
In addition, according to the present invention, there is provided an image forming apparatus in which a process cartridge formed by integrating process elements including a photosensitive drum into a unit is detachably attachable, comprising: detecting means for detecting an edge of image data; image data correcting means for correcting the image data inside the edge detected by the detecting means so as to decrease a density of the image data; and forming means for forming an electrostatic latent image on a photosensitive body on the basis of the corrected image data, wherein the process cartridge includes storage means for storing information about the process cartridge, and wherein a correction condition for image data corrected by the image data correcting means changes in accordance with the information stored in the storage means.
Furthermore, according to the present invention, there is provided an image forming apparatus in which a process cartridge formed by integrating process elements including a photosensitive drum into a unit is detachably attachable, comprising: correcting means for correcting image data; forming means for forming an electrostatic latent image on a photosensitive body included in the process cartridge on the basis of the image data corrected by the correcting means; and acquiring means for acquiring an instruction value for giving an instruction to correct a density contrast; wherein the process cartridge includes storage means for storing information about the process cartridge, and wherein the image data correcting means corrects image data on the basis of the instruction value acquired by the acquiring means and the information stored in the storage means.
Moreover, according to the present invention, there is provided an image forming method of forming an image on a photosensitive body included in a process cartridge formed by integrating process elements into a unit, comprising: an image data correcting step of correcting image data; a forming step of forming an electrostatic latent image on the photosensitive body included in the process cartridge on the basis of the image data corrected in the correcting step; and an acquiring step of acquiring an instruction value for giving an instruction to correct a density contrast, wherein the process cartridge includes storage means for storing information about the process cartridge, and wherein in the image data correcting step, the image data is corrected on the basis of the instruction value acquired in the acquiring step and the information stored in the storage means.